1. Field of the Invention
The present invention relates to deposition of oxide and oxynitride films and, in particular, to deposition of oxide and oxynitride films by pulsed DC reactive sputtering.
2. Discussion of Related Art
Deposition of insulating materials and especially optical materials is technologically important in several areas including production of optical devices and production of semiconductor devices. In semiconductor devices, doped alumina silicates can be utilized as high dielectric insulators.
The increasing prevalence of fiber optic communications systems has created an unprecedented demand for devices for processing optical signals. Planar devices such as optical waveguides, couplers, splitters, and amplifiers, fabricated on planar substrates, like those commonly used for integrated circuits, and configured to receive and process signals from optical fibers are highly desirable. Such devices hold promise for integrated optical and electronic signal processing on a single semiconductor-like substance.
The basic design of planar optical waveguides and amplifiers is well known, as described, for example, in U.S. Pat. Nos. 5,119,460 and 5,563,979 to Bruce et al., 5,613,995 to Bhandarkar et al., 5,900,057 to Buchal et al., and 5,107,538 to Benton et al., to cite only a few. These devices, very generally, include a core region, typically bar shaped, of a certain refractive index surrounded by a cladding region of a lower refractive index. In the case of an optical amplifier, the core region includes a certain concentration of a dopant, typically a rare earth ion such as an erbium or praseodymium ion which, when pumped by a laser, fluoresces, for example, in the 1550 nm and 1300 nm wavelength ranges used for optical communication, to amplify the optical signal passing through the core.
As described, for example in the patents by Bruce et al., Bhandarkar et al, and Buchal et al., planar optical devices may be fabricated by process sequences including forming a layer of cladding material on a substrate; forming a layer of core material on the layer of cladding mater; patterning the core layer using a photolighotgraphic mask and an etching process to form a core ridge; and covering the core ridge with an upper cladding layer.
The performance of these planar optical devices depends sensitively on the value and uniformity of the refractive index of the core region and of the cladding region, and particularly on the difference in refractive index, Δn, between the regions. Particularly for passive devices such as waveguides, couplers, and splitters, Δn should be carefully controlled, for example to values within about 1%, and the refractive index of both core and cladding need to be highly uniform, for some applications at the fewer than parts per thousand level. In the case of doped materials forming the core region of planar optical amplifiers, it is important that the dopant be uniformly distributed so as to avoid non-radiative quenching or radiative quenching, for example by upconversion. The refractive index and other desirable properties of the core and cladding regions, such as physical and chemical uniformity, low stress, and high density, depend, of course, on the choice of materials for the devices and on the processes by which they are fabricated.
Because of their optical properties, silica and refractory oxides such as Al2O3, are good candidate materials for planar optical devices. Further, these oxides serve as suitable hosts for rare earth dopants used in optical amplifiers. A common material choice is so-called low temperature glasses, doped with alkali metals, boron, or phosphorous, which have the advantage of requiring lower processing temperatures. In addition, dopants are used to modify the refractive index. Methods such as flame hydrolysis, ion exchange for introducing alkali ions in glasses, sputtering, and various chemical vapor deposition processes (CVD) have been used to form films of doped glasses. However, dopants such as phosphorous and boron are hygroscopic, and alkalis are undesirable for integration with electronic devices. Control of uniformity of doping in CVD processes can be difficult and CVD deposited films can have structural defects leading to scattering losses when used to guide light. In addition, doped low temperature glasses may require further processing after deposition. A method for eliminating bubbles in thin films of sodium-boro-silicate glass by high temperature sintering is described, for example, in the '995 patent to Bhandarkar et al.
Typically, RF sputtering has been utilized for deposition of oxide dielectric films. However, RF sputtering utilizes ceramic targets which are typically formed of multiple smaller tiles. Since the tiles can not be made very large, there may be a large problem of arcing between tiles and therefore contamination of the deposited film due to this arcing. Further, the reactors required for RF sputtering tend to be rather complicated. In particular, the engineering of low capacitance efficient RF power distribution to the cathode is difficult in RF systems. Routing of low capacitance forward and return power into a vacuum vessel of the reaction chamber often exposes the power path in such a way that diffuse plasma discharge is allowed under some conditions of impedance tuning of the matching networks.
Therefore, there is a need for new methods of depositing oxide and oxynitride films and for forming planar optical devices.